PGDLLIMPORT-ify MainLWLockArray, ProcDiePending, proc_exit_inprogress.

These are needed in HEAD to make assorted contrib modules build on Windows.
Now that all the MSVC and Mingw buildfarm members seem to be on the same
page about the need for them, we can have some confidence that future
problems of this ilk will be detected promptly; there seems nothing more
to be learned by delaying this fix further.

I chose to mark QueryCancelPending as well, since it's easy to imagine code
that wants to touch ProcDiePending also caring about QueryCancelPending.